Optical communications systems facilitate high-speed and high-bandwidth data transmission over short and long distances (e.g., from a few hundred meters within a datacenter to thousands of meters of underwater fiber). One type of optical communication system for transmitting data over long distances is known as a long-haul optical communication system. In long-haul optical communication systems, data rates are increased by employing coherent optical transmissions, in which data may be modulated onto amplitudes, phases, and/or polarization components of an optical carrier signal. In these long-haul optical communication systems, the performance may be limited by fiber optic impairments such as chromatic dispersion (CD), local oscillator frequency offset (LOFO), polarization mode dispersion (PMD), phase noise, and non-linear effects.
For example, CD may cause different spectral components (e.g., wavelengths) in a light signal to travel through an optical fiber at different speeds and arrive at a receiver at different time instants, and thus may broaden the light pulses that carry the data and lead to ISI. As another example, LOFO is a frequency offset between a received data carrier and a local oscillator in a receiver. For lasers used in coherent optical communication systems, a LOFO can reach up to about +/−3 gigahertz (GHz) and can create a linear ramp in the phase of mixed signals. The combination of CD and LOFO, if left uncompensated for, can significantly hinder and even prevent proper downstream processing of the data transmitted through the optical communication systems. In coherent optical communications systems that use a multiple-subcarrier-based transmission (e.g., orthogonal frequency-division multiplexing (OFDM)), accurate compensation for CD and LOFO can take on an increased importance when compared to single-carrier-based transmission. For example, when using multiple-subcarrier-based transmission, an amount of time available for determining compensation for the CD and LOFO may be limited. Accordingly, a method for quickly and accurately determining compensation for CD and LOFO in coherent optical communications systems may be desirable.